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• Comparison of the nominal and quaternion attitude realisation of TOPEX/Poseidon. • Angular differences in roll, pitch, yaw, and the solar array rotation angle. • Influence on the radial position of the altimeter antenna due to mismodelling. • Similar results with slight improvements for the quaternion attitude realisation. • Change in the radial position up to 2.5 cm in case of mismodelling in roll/pitch. The altimetry mission TOPEX/Poseidon was operational between 1992 and 2005 and provided innovative insight into global and regional sea level changes and their variability. The correct modelling of the spacecraft orbit in precise orbit determination requires detailed information on the attitude of the spacecraft body and the solar array. Since the availability of attitude observations in form of quaternion data for this mission is limited, the nominal orientation model has to be used mainly. In this study, we compare the nominal and quaternion-based approaches of the TOPEX/Poseidon attitude realisation and validate the nominal model w.r.t. the observed attitude. We found good agreement between both approaches in the spacecraft-related roll, pitch, yaw, and solar array angles with slight differences of up to 0.31 ° on average. The best accordance is obtained in the yaw angles. A general angular offset of 0.12 ° and - 0.19 ° on average is found in the pitch and solar array angle differences, respectively. Parameters estimated in the orbit computation process also reflect the good agreement between both orientation models with slightly better results when using the observed attitude. However, in some intervals, the nominal model lacks detailed information about the correct attitude. These can be adjusted using the information from the quaternion data. Such intervals indicate off-nominal spacecraft attitude events like drifts or offsets. Purely geometrically analysed, mispointing in the roll and pitch components of about 0.25 ° results in a change of the altimeter phase centre position in the radial direction of about 7 mm, which directly impacts the sea-level heights determined from altimetry measurements. [ABSTRACT FROM AUTHOR]

Coastal zones supporting diurnal shelf waves near the South Kuril Islands were determined based on the analysis of small-scale amplitude and phase variations of diurnal O1 and K1 tidal waves estimated from along-track TOPEX/Poseidon altimetry data. The amplitudes and phases of these waves were revealed not only along the original tracks (1992–2002), but also along those shifted by half the intertrack distance (2002–2005). This made it possible to more accurately determine the zone supporting diurnal shelf waves. It was found that diurnal shelf waves in the form of baroclinic coastal trapped modes strongly influence tidal motions on the oceanic shelf of Urup and Iturup islands. Two wavelengths fit in this section of the shelf (one pair of quasi-stationary eddies is localized on the shelf of Urup Island and a second one on the shelf of Iturup Island). This is an unexpected result, as barotropic shelf modes usually form one pair of eddies (for example, on the northeastern shelf of Sakhalin Island, on the northern shelf of Hokkaido Island and on the oceanic shelf of the North Kuril Islands). We found that the zone of K1 and O1 tidal waves start from Chirpoi Island, which means that shelf waves are generated as a result of the diffraction of a large-scale Kelvin tidal wave in the deep Bussol Strait. The existence of the lower frequency O1 tidal shelf wave was found not only on the shelf of Urup and Iturup islands, but also on the shelf of Shikotan Island, which is consistent with the earlier results of Yefimov and Rabinovich (1980). Diurnal shelf waves cause significant increases in the O1 and K1 amplitudes on the coast of Iturup Island (especially at its southern part) and on Urup Island in comparison with the open ocean, and produce strong instrumentally-observed diurnal tidal currents in these regions. Baroclinic diurnal shelf (coastal trapped) waves also contribute to vertical movements, forcing water temperature variations at sufficiently great depths, which can have a significant impact on the living conditions of benthic communities. [ABSTRACT FROM AUTHOR]

Radar altimetry has been used to monitor sea level changes and ice sheet elevation changes for decades. Over mountain glaciers, radar altimetry has limited applications due to contaminated waveforms caused by complex glacier surfaces and steep terrains. In this study, we develop a glacier‐threshold method (GTM) to determine glacier elevation changes in Alaska and Yukon. The GTM retracks waveforms, reduces terrain effect and detects invalid elevation observations from the TOPEX/Poseidon (T/P) and Jason‐2 (J2) altimeters, resulting in an average usable rate of 35% from original altimeter‐measured heights. The selected measurements are used to construct time series of glacier elevation changes over 1993–2002 (T/P) and 2008–2016 (J2) at 47 sites. A crossover analysis, validation by airborne laser altimetry observations, and comparisons with recent studies confirm the estimated glacier elevation changes. Our findings suggest that the thinning of glaciers in Alaska and Yukon has been ubiquitous in recent years. The site near Walsh Glacier has the highest thinning rate of −5.71 ± 0.09 m/yr, followed by Chitina Glacier at −4.51 ± 0.21 m/yr. Only 17% of sites show glacier thickening due to surges and mass accumulations. Using a sophisticated data processing algorithm like the GTM, we show that altimeter data from the repeat TOPEX‐Jason missions can be used to monitor long‐term glacier elevation changes at inaccessible spots, creating an additional value for altimeter missions originally purposed for monitoring long‐term sea level change. Plain Language Summary: Satellite radar altimetry is an important observation technology to determine changes in surface elevations. However, altimeter measurement accuracy is very sensitive to terrain complexity, especially over mountain glaciers. Here, we present a glacier‐threshold method (GTM) to investigate the glacier elevation changes in Alaska and Yukon, using TOPEX/Poseidon (T/P) and Jason‐2 (J2) altimeter measurements. Internal and external measurement assessments show that the GTM enables us to retrieve precise changes in glacier elevations and improve the usable rate of original radar elevation observations. During the recent study period, the vast majority of Alaska and Yukon glaciers' elevations are rapidly declining. But in fewer glacier accumulation areas and over glaciers with active surface processes, the glacier elevations are rising. Environmental factors analysis shows that Pacific decadal oscillation, temperature, precipitation, surges, altitudes and locations of glaciers are responsible for the asynchronously spatiotemporal glacier elevation changes. All these findings from the repeat‐track radar altimetry and the GTM help to improve our knowledge about Alaska and Yukon glacier processes. Key Points: Glacier‐threshold method retrieves Alaska and Yukon glacier elevation changes using approximately 35% of repeated altimeter measurementsAlaska and Yukon have recently experienced notable glacier thinning, with the highest rate of −5.71 ± 0.09 m/yr near Walsh GlacierPacific decadal oscillation, surges, altitudes, and glacier locations affect Alaska and Yukon glacier elevation change pattern [ABSTRACT FROM AUTHOR]

• New approach of a system- and station-dependent TOPEX/Poseidon SLR measurement correction function. • It is based on normal point observations and improves the mean orbit RMS from 33 cm to 1.97 cm. • The correction function and the station correction parameters are publicly available. The TOPEX/Poseidon (T/P) altimetry mission with its main objectives to monitor variations of the global and regional sea level as well as ocean circulation is a milestone in Earth observation. The spacecraft was launched in 1992 and it is the predecessor mission of the Jason series and Sentinel-6A. For laser ranging measurements from the Earth to the spacecraft, T/P was equipped with a non-ideally designed annular retroreflector array. Its large dimensions of over 160 cm in diameter caused huge optical phase centre variations which limit the orbit accuracy. In this study, we developed a continuous, analytical correction function that counteracts both, phase centre variations at the spacecraft and station-related laser ranging measurement errors such as range biases. For most Satellite Laser Ranging (SLR) ground stations that tracked the T/P spacecraft, an individual set of six correction parameters is estimated. The parameters are valid for the entire mission or, in some cases, for a defined period. The developed function uses the observation's viewing angles to determine a correction value which is added to the range measurement. Applying the measurement correction reduces the overall T/P mission root mean square fit of SLR residuals from 33.78 cm to 1.97 cm (1.59 cm for SLR core stations). External orbits based on the joint analysis of two different space-geodetic techniques are used to validate the quality of our improved orbit solution. The comparisons show good agreement. The mean values of the radial, transverse, and normal components differ by 0.0 , - 0.1 , and - 0.2 cm, respectively. To investigate the impact of the corrected orbit on sea level computations, a single-satellite crossover analysis is performed. When using the corrected measurements, the standard deviation of crossover differences reduces from 16 cm to 6 cm. The computed coefficients of the measurement correction function are provided publicly and can be used in any software package to obtain T/P orbits. [ABSTRACT FROM AUTHOR]

The tumbling motion of defunct satellite TOPEX/Poseidon (T/P) is governed by the μ-scale Solar Radiation Pressure (SRP) torque that spun up the satellite from the initial, nadir-pointing to the current fast spinning state [1]. In this paper, we advance the methods of Quanta Photogrammetry (QPM) in an effort to determine satellite attitude parameters – the inertial orientation of the body angular momentum vector L , the rotational phase and rate as well as the object pole coordinates with respect to L ; the determined pole offset is 3.52°. The presented data support the hypothesis that the full characteristics of the satellite attitude dynamics can be obtained from a single radiant photogram measured by the photon-counting system of the Graz Satellite Laser Ranging (SLR) Observatory during an overhead pass. The analysis of the observed radiant signature of T/P gives the elevation angle of the normal vector to the solar array front surface at −1.7°, RMS = 0.63°, in the spacecraft body-centered reference frame. This information, along with the pole position, is used to model the SRP forces and torques on the satellite and determine the T/P moment of inertia I = 69728 , σ I = 75.2 [ k g m 2 ] , by best-fitting long-term spin simulation to the observed trend. • Quanta Photogrammetry allows for the full attitude state reconstruction of a tumbling space object. • The pole coordinates of tumbling TOPEX/Poseidon with respect to its rotational angular momentum vector are determined. • The rotational inertia of TOPEX/Poseidon is obtained with the physics-based force and torque simulations. [ABSTRACT FROM AUTHOR]

Zhari Namco, a large lake in the Tibetan Plateau (TP), is sensitive to climate and environmental change. However, it is difficult to retrieve accurate and continuous lake levels for Zhari Namco. A robust strategy, including atmospheric delay correction, waveform retracking, outlier deletion, and inter-satellite adjustment, is proposed to generate a long-term series of lake levels for Zhari Namco through multi-altimeter data. Apparent biases are found in troposphere delay correction from different altimeter products and adjusted using an identical model. The threshold (20%) algorithm is employed for waveform retracking. The two-step method combining a sliding median filter and 2σ criterion is used to eliminate outliers. Tandem mission data of altimeters are used to estimate inter-satellite bias. Finally, a 27-year-long lake level time series of Zhari Namco are constructed using the TOPEX/Poseidon-Jason1/2/3 (T/P-Jason1/2/3) altimeter data from 1992 to 2019, resulting in an accuracy of 10.1 cm for T/P-Jason1/2/3. Temperature, precipitation, lake area, equivalent water height, and in situ gauge data are used for validation. The correlation coefficient more than 0.90 can be observed between this result and in situ gauge data. Compared with previous studies and existing database products, our method yields sequences with the best observational quality and the longest continuous monitoring in Zhari Namco. The time series indicates that the lake level in Zhari Namco has increased by ∼ 5.7 m, with an overall trend of 0.14 ± 0.01 m/yr, showing a fluctuating rate (1992–1999: −0.25 ± 0.05 m/yr, 2000–2008: 0.26 ± 0.04 m/yr, 2009–2016: −0.05 ± 0.03 m/yr, 2017–2019: 1.34 ± 0.34 m/yr). These findings will enhance the understanding of water budget and the effect of climate change in the TP.

Accurate knowledge of the rotational dynamics of a large space debris is crucial for space situational awareness (SSA), whether it be for accurate orbital predictions needed for satellite conjunction analyses or for the success of an eventual active debris removal mission charged with stabilization, capture and removal of debris from orbit. In this light, the attitude dynamics of an inoperative satellite of great interest to the space debris community, the joint French and American spacecraft TOPEX/Poseidon, is explored. A comparison of simulation results with observations obtained from high-frequency satellite range measurements is made, showing that the spacecraft is currently spinning about its minor principal axis in a stable manner. Predictions of the evolution of its attitude motion to 2030 are presented, emphasizing the uncertainty on those estimates due to internal energy dissipation, which could cause a change of its spin state in the future. The effect of solar radiation pressure and the eddy-current torque are investigated in detail, and insights into some of the satellite's missing properties are provided. These results are obtained using a novel, open-source, coupled orbit-attitude propagation software, the Debris SPin/Orbit Simulation Environment (D-SPOSE), whose primary goal is the study of the long-term evolution of the attitude dynamics of large space debris. [ABSTRACT FROM AUTHOR]

For decades, satellite altimetry is providing reliable data on the sea level, surface currents, tides, planetary and wind waves and sea ice. We propose a potential enhancement of collecting special data subsets for a better temporal and spatial sampling of dynamics of the Sub-Polar seas. We show that peculiarities of the orbits of the TOPEX/Poseidon and Jasons’ missions generate a regular grid of crossovers which locations are fixed in space and time of the orbit cycles. Two types of these special crossovers are specified: self-crossovers (SC) occurring as intersections of tracks of the same satellite and inter-crossovers (IC) appearing during the phases of interleaved orbits when tracks of two different satellites are crossing. The time delay between two consecutive measurements in the high-latitude sites is less than 2 h for SC and 1 h for IC, thus providing a ground for monitoring large-scale currents (including current direction) and wind waves. The sets of the special crossovers can be regarded as networks of virtual ocean buoys. The Norwegian Sea case study illustrates the value of the special crossovers for the Sub-Polar seas and the feasibility of building the networks of SC and IC in other areas of the World Ocean.

Satellite altimetry has been widely used for inland water monitoring and modeling in the present decade. In this paper, the availability, data quality, and quantity of a number of spaceborne radar altimetry missions (including Topex/Poseidon (TP), ERS-2, Envisat, Jason-1/−2/−3, CryoSat-2, Saral, and Sentinel-3) are investigated. This is, to the best of our knowledge, the first paper to investigate the influences of onboard trackers, satellite flying directions, and land contamination on Level-1 waveforms and Level-2 measurements over large mountainous lakes. The analyses were conducted over eight Tibetan lakes, which are of varying sizes from ca. 93 to 1046 km2 and are sampled by multiple altimeters. From the results presented in this paper, it is demonstrated that the previous generation of altimeters, such as TP, Jason-1, ERS-2, and Envisat, shows a higher percentage of data unavailability and lower valid observation rates. Investigations of Level-1 waveforms show onboard tracker failing in locking on the lake surface, resulting in data loss or invalid measurements. In addition, the onboard tracking systems are sometimes unstable, resulting in larger along-track standard deviations (SD) (SD > 15 cm). In contrast, the recent generation of altimeters, i.e., Jason-3 and Sentinel-3 with open-loop tracking mode, significantly increases the data availability (> 90% passes and > 60% individual observations) and delivers higher precision measurements (SD around 10 cm). Moreover, the waveform leading edge is very consistent, indicating more stable onboard tracking. Higher quality Jason-3 and Sentinel-3 data allow monitoring of sub- and quasi-monthly lake variations, which are not revealed by e.g., TP and ERS-2 data due to poorer signal-to-noise ratios. Exceptionally, Saral Ka-band altimeter, operated in closed-loop tracking mode, is less affected by flying directions and land contamination (topography and landscapes), delivering high quality measurements (SD around 7 cm for 40 Hz). This study provides a comprehensive comparison of multiple altimeters in monitoring lakes in mountainous areas, and demonstrates the substantial improvements achieved in the past decades. However, care has to be taken when merging data from old generation altimetry missions over mountainous lakes. The lessons learned from these Tibetan lakes can be taken across to other mountain lakes. • Comprehensive assessment of multiple altimeters performance over Tibetan lakes. • Investigation of influences of onboard trackers on Level-1 waveforms and Level-2 measurements. • Onboard tracking failure is largely responsible for erroneous measurements contained in the GDR. • Distinctive improvements (∼ 100% data availability & < 10 cm precision) achieved during 1992–2021. [ABSTRACT FROM AUTHOR]

The calibration of satellite radar altimetry has been extremely important for altimetry community and studying sea level changes. The main purpose of this contribution is to provide ongoing absolute calibration of altimeter bias near the Southern seas of Iran using the Iranian tide gauge network that equipped with GPS receivers to measure the sea surface heights synchronously in the same geocentric reference frame as the corresponding altimetry records. The sea level time series of coastal tide gauges have been used to estimate the bias, drift and annual/semiannual constituents of altimeter range measurements using (i) linear regression and (ii) combination of linear regression and harmonic analysis. To this end, three Iranian tide gauges located at Bushehr, Bandar Abbas and Chahbahar ports as well as Geophysical Data Records (GDR) products of Topex/Poseidon, Jason-1and Jason-2 have been considered. The numerical results have indicated that the mean absolute biases of Topex/Poseidon, Jason-1 and Jason-2 are about –26.23, 120.21 and 205.17 mm, respectively. The reliability of method has been assessed via GPS vessel at the altimeter bin nearby the Bushehr tidal stations. The presented method is viable to perfectly estimate the systematic errors, and as such, it can address the demands of high-accurate applications.

Satellite altimetry has been successfully applied to monitoring water level variation of global lakes. However, it is still difficult to retrieve accurate and continuous observations for most Tibetan lakes, due to their high altitude and rough terrain. Aiming to generate long-term and accurate lake level time series for the Tibetan lakes using multi-altimeters, we present a robust strategy including atmosphere delay corrections, waveform retracking, outlier removal and inter-satellite bias adjustment. Apparent biases in dry troposphere corrections from different altimeter products are found, and such correctios must be recalculated using the same surface pressure model. A parameter is defined to evaluate the performance of the retracking algorithm. The ICE retracker outperforms the 20% and 50% threshold retrackers in the case of Ngangzi Co, where a new wetland has been established. A two-step algorithm is proposed for outlier removal. Two methods are adopted to estimate inter-satellite bias for different cases of with and without overlap. Finally, a 25-year-long lake level time series of Ngangzi Co are constructed using the TOPEX/Poseidon-family altimeter data from October 1992 to December 2017, resulting in an accuracy of ~17 cm for TOPEX/Poseidon and ~10 cm for Jason-1/2/3. The accuracy of retrieved lake levels is on the order of decimeter. Because of no gauge data available, ICESat and SARAL data with the accuracy better than 7 cm are used for validation. A correlation more than 0.9 can be observed between the mean lake levels from TOPEX/Poseidon-family satellites, ICESat and SARAL. Compared to the previous studies and other available altimeter-derived lake level databases, our result is the most robust and has resulted in the maximum number of continuous samples. The time series indicates that the lake level of Ngangzi Co increased by ~8 m over 1998–2017 and changed with different rates in the past 25 years (-0.39 m/yr in 1992–1997, 1.03 m/yr in 1998–2002 and 0.32 m/yr in 2003–2014). These findings will enhance the understanding of water budget and the effect of climate change.

Lake levels in the Qinghai-Tibet Plateau (QTP) provide valuable records for climate change studies. We use two decades of measurements (January 1993-December 2014) from the TOPEX/Poseidon (T/P)-family satellite altimeters (T/P, Jason-1 and -2) to detect lake level variations at 23 lakes along their repeat ground tracks every 10 days. We employ an optimal processing technique to obtain quality measurements, including outlier detection, averaging and filtering. The lake level accuracies are improved by subwaveform retracking. Jason-1 delivers few measurements after waveform retracking and a cluster classification at most lakes. From January 1993 to December 2014, most lake levels in eastern Tibet rose, while those in western Tibet declined. In Qinghai, lake levels dropped before 2005 and then rose afterwards, coinciding with the measure in 2005 that protects the Qinghai ecosystem (e.g., grassland conservation). The overall pattern of lake level change in the QTP is largely affected by monsoons and lake locations. Most lake levels show clear annual and inter-annual oscillations. Certain lakes show alternating level highs and lows in the same seasons and varying amplitudes of annual oscillations due to lake level changes. We detect a sudden rise of lake level by 7 m caused by floods, varying lake level trends associated with the 1997-98 El Niño and other factors, and persistently rising and declining lake levels associated with the long-term precipitation trends in the QTP. The T/P-family satellites will continue to monitor lake levels here as long as the sea level monitoring program lasts, collecting a long-term climate record at highlands echoing sea level change. [ABSTRACT FROM AUTHOR]

To take maximum advantage of the increasing Global Navigation Satellite Systems (GNSS) data to improve the accuracy and resolution of global ionospheric TEC map (GIM), an approach, named Spherical Harmonic plus generalized Trigonometric Series functions (SHPTS), is proposed by integrating the spherical harmonic and the generalized trigonometric series functions on global and local scales, respectively. The SHPTS-based GIM from January 1st, 2001 to December 31st, 2011 (about one solar cycle) is validated by the ionospheric TEC from raw global GPS data, the GIM released by the current Ionospheric Associate Analysis Center (IAAC), the TOPEX/Poseidon satellite and the DORIS. The present results show that the SHPTS-based GIM over the area where no real data are available has the same accuracy level (approximately 2-6 TECu) to that released by the current IAAC. However, the ionospheric TEC in the SHPTS-based GIM over the area covered by real data is more accurate (approximately 1.5 TECu) than that of the GIM (approximately 3.0 TECu) released by the current IAAC. The external accuracy of the SHPTS-based GIM validated by the TOPEX/Poseidon and DORIS is approximately 2.5-5.5 and 1.5-4.5 TECu, respectively. In particular, the SHPTS-based GIM is the best or almost the best ranked, along with those of JPL and UPC, when they are compared with TOPEX/Poseidon measurements, and the best (in addition to UPC) when they are validated with DORIS data. With the increase in the number of GNSS satellites and contributing stations, the performance of the SHPTS-based GIM can be further improved. The SHPTS-based GIM routinely calculated using global GPS, GLONASS and BDS data will be found at the website . [ABSTRACT FROM AUTHOR]

We assess four different global ionospheric maps (GIMs) over the area of China based on internal consistency (W.r.t.GNSS-derived total electron content (TEC)) and external accuracy (W.r.t.Topex/Poseidon-derived TEC). The results of relevance would serve as references for single-frequency GNSS Positioning, Navigation and Timing (PNT) users to flexibly determine which GIM is to be based on to get the more efficient ionospheric delay corrections service. Performance of these four GIMs sources are validated during high level (2003) as well as low level (2009) solar activity and even 10 years data is tested against GNSS-derived TEC over China and its neighborhood. Results show that UPC GIMs outperform all the rest of GIMs when ionospheric gradients are large, and there is marginally difference in low solar activity or middle latitude among these GIMs since 2006. Hence, we suggest that the UPC GIMs should be used in solar maximum and low latitude. It is also reasonable to apply any GIMs in low solar activity and middle latitude. [ABSTRACT FROM AUTHOR]

The climatology of significant wave height (SWH) and sea surface wind speed are matters of concern in the fields of both meteorology and oceanography because they are very important parameters for planning offshore structures and ship routings. The TOPEX/Poseidon altimeter, which collected data for about 13 years from September 1992 to October 2005, has measured SWHs and surface wind speeds over most of the world's oceans. In this paper, a study of the global spatiotemporal distributions and variations of SWH and sea surface wind speed was conducted using the TOPEX/Poseidon altimeter data set. The range and characteristics of the variations were analyzed quantitatively for the Pacific, Atlantic, and Indian oceans. Areas of rough waves and strong sea surface winds were localized precisely, and the correlation between SWH and sea surface wind speed analyzed. [ABSTRACT FROM AUTHOR]

For decades, satellite altimetry is providing reliable data on the sea level, surface currents, tides, planetary and wind waves and sea ice. We propose a potential enhancement of collecting special data subsets for a better temporal and spatial sampling of dynamics of the Sub-Polar seas. We show that peculiarities of the orbits of the TOPEX/Poseidon and Jasons' missions generate a regular grid of crossovers which locations are fixed in space and time of the orbit cycles. Two types of these special crossovers are specified: self-crossovers (SC) occurring as intersections of tracks of the same satellite and inter-crossovers (IC) appearing during the phases of interleaved orbits when tracks of two different satellites are crossing. The time delay between two consecutive measurements in the high-latitude sites is less than 2 h for SC and 1 h for IC, thus providing a ground for monitoring large-scale currents (including current direction) and wind waves. The sets of the special crossovers can be regarded as networks of virtual ocean buoys. The Norwegian Sea case study illustrates the value of the special crossovers for the Sub-Polar seas and the feasibility of building the networks of SC and IC in other areas of the World Ocean. [ABSTRACT FROM AUTHOR]

Satellite altimetry has been successfully applied to monitoring water level variation of global lakes. However, it is still difficult to retrieve accurate and continuous observations for most Tibetan lakes, due to their high altitude and rough terrain. Aiming to generate long-term and accurate lake level time series for the Tibetan lakes using multi-altimeters, we present a robust strategy including atmosphere delay corrections, waveform retracking, outlier removal and inter-satellite bias adjustment. Apparent biases in dry troposphere corrections from different altimeter products are found, and such correctios must be recalculated using the same surface pressure model. A parameter is defined to evaluate the performance of the retracking algorithm. The ICE retracker outperforms the 20% and 50% threshold retrackers in the case of Ngangzi Co, where a new wetland has been established. A two-step algorithm is proposed for outlier removal. Two methods are adopted to estimate inter-satellite bias for different cases of with and without overlap. Finally, a 25-year-long lake level time series of Ngangzi Co are constructed using the TOPEX/Poseidon-family altimeter data from October 1992 to December 2017, resulting in an accuracy of ~17 cm for TOPEX/Poseidon and ~10 cm for Jason-1/2/3. The accuracy of retrieved lake levels is on the order of decimeter. Because of no gauge data available, ICESat and SARAL data with the accuracy better than 7 cm are used for validation. A correlation more than 0.9 can be observed between the mean lake levels from TOPEX/Poseidon-family satellites, ICESat and SARAL. Compared to the previous studies and other available altimeter-derived lake level databases, our result is the most robust and has resulted in the maximum number of continuous samples. The time series indicates that the lake level of Ngangzi Co increased by ~8 m over 1998–2017 and changed with different rates in the past 25 years (-0.39 m/yr in 1992–1997, 1.03 m/yr in 1998–2002 and 0.32 m/yr in 2003–2014). These findings will enhance the understanding of water budget and the effect of climate change. [ABSTRACT FROM AUTHOR]